Conventionally, in an electronic device having an imaging function such as a digital still camera or a digital video camera, for example, a solid state imaging element such as a charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS) image sensor is used. The solid state imaging element includes a pixel in which a photodiode (PD) that performs photoelectric conversion and a plurality of transistors are combined, and an image is constructed on the basis of pixel signals outputted from a plurality of pixels arranged in a planar way. The pixel signals outputted from the pixels are, for example, A/D-converted by a plurality of analog-to-digital (A/D) converters arranged individually for columns of pixels, and are outputted.
For example, in Patent Literature 1, a solid state imaging element including an A/D converter configured to include a comparator that compares a saw wave-like ramp signal and a pixel signal and a counter is disclosed. Further, in Patent Literature 2, a solid state imaging element in which, in an A/D conversion system called a single-slope integration type, gain adjustment can be made by changing the slope of a reference signal supplied to a comparison circuit is disclosed.
In the imaging device, for example, when the subject is bright, the amount of light is reduced by an aperture mechanism placed in an optical system that collects light to the solid state imaging element. However, in an imaging device including an optical system with a simple aperture mechanism, when moving images are captured under a high illuminance, the exposure time for one image (one frame) of the images constituting the moving images is extremely short. Hence, the continuity between frames is impaired, and moving images having a feeling of skipping are captured.
In contrast, conventionally, for example, electronic ND functions that reduce the light electronically are proposed, such as a configuration in which a variable capacitance is provided in a pixel to make the conversion efficiency variable and a configuration in which quadra-coding that uses 4 pixels as 1 pixel is used and only the single pixels are accessed.